change group and user of process

I want a root user to be able to run my program like:

myprog -u someUser -g someGroup

and then have the program switch to that user and group when it starts running.  What functions do I need to use to program this in C?
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lwinkenbAsked:
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g0rathConnect With a Mentor Commented:
also for the argument parsing, have you looked up getopt()

man 3 getopt

int main( int argc, char **argv )
{

int c;

while (1)
{
    c = getopt(argc,argv,"u:g:")
    if (c == -1)
         break;

    switch (c)
    {
         case 'u':
             printf ("%s\n",optarg); // your user
             break;
         case 'g':
             printf ("%s\n",optarg); // your group
             break;
         
    }
}


return 0;
}
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g0rathCommented:
look up the man pages for...
User Identiy functions
setuid
seteuid
setreuid
setresuid


Group identity
setgid
setegid
setregid


usage like...

seteuid(uid_of_bob)
// Do stuff as bob
seteuid(0)
// Switch back to Root
seteuid(uid_of_frank)
// Do stuff as frank
// etc...
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lwinkenbAuthor Commented:
Thanks a lot
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